Viscosity of a liquid is an important parameter which helps manufacturers predict how a material will behave in the real world and this latest blog introduces a new method from Hintermüller et al. , on measuring viscosity using Cellix’s ExiGo syringe pumps.
Unfortunately, viscosity is not so easy to measure…but first, what is viscosity and why is it important?
What is Viscosity?
If you want to look at the real behaviour of fluids, then you need to understand viscosity. There are various definitions of viscosity including the internal friction of a liquid; a fluid’s ability to resist flow or resistance to deformation under shear stresses. As a formula, it is described as:
Viscosity = shear stress / shear rate
When describing viscosity in terms of a liquid, you may hear people describe a liquid as “thin” which is more water-like and these liquids typically have a low viscosity. You’ve probably heard the phrase “thinning out paint” and this is where paint is watered down. In contrast, “thick” liquids such as honey or syrup are those with high viscosity.
Viscosity measurements are used in many industries including:
Food – to maximise production efficiency and cost effectiveness; e.g. how toothpaste flows out of a tube or ketchup out of a jar.
Oil – to determine the effectiveness of lubricating oil or in the case of crude oil, viscosity determines our ability to pump it out of the ground.
Cosmetics – to determine the texture, feel and flow of cosmetic products.
Chemical and biological processes monitoring.
So, what’s the problem with measuring viscosity?
One of the first methods for measuring viscosity was introduced in the 1930s and this involved measuring the time it takes for a volume of fluid to flow under gravity through a calibrated glass capillary. Since then, other methods have been introduced including using rotating concentric cylinders or plate configurations. But one of the main challenges is achieving high accuracy and precision. Why is this important? Because even an error of just 1% can cause blend adjustments that easily result in increasing product cost by a penny per gallon. In economic terms, this can could cost a large lubricant manufacturer $1 million or more in lost revenue per year, . Also, many existing commercial methods lack the possibility for in-line measurements and require large sample volumes; the latter rendering them difficult to use for biological samples where often only small sample volumes are available. As a result, in recent years, microfluidic solutions have been developed which include methods to detect the interface position in a microfluidic channel between co-flowing liquids (one sample liquid and one reference liquid). The problem with this is that optical methods are usually used to determine this interface position – this requires colouring the liquids or adding tracer particles to increase optical contrast.
A potential solution to these methods is presented by Hintermüller et al. which describes a microfluidic viscometer with an integrated capacitive sensor. The capacitive measurement is achieved by interdigitated electrodes which are screen- printed onto the bottom of the chip.
Advantages to this method include:
No addition of coloured dyes or tracer particles required
Both fluids may be conductive and/or insulating, as long as they have a dielectric constant.
Electrochemical effects are reduced as the electrodes are not directly in contact with the fluids.
Suitable for measurement of liquids with low viscosity, .
Optical inspection is still possible if required as the electrodes are only positioned on the bottom of the microfluidic chip.
How does it work?
The reference fluid has a viscosity “R” while the sample fluid to be measured has a viscosity “S”. Exploiting the laminar flow of two fluids in a microfluidic channel, an interface is established between the two fluids, but the position of this interface will move depending on the viscosity and the flow rates of the fluids. If the viscosity of the reference fluid, R, is known and the flow rates of both fluids are known, then the viscosity of the sample fluid may be calculated. To illustrate this, see Fig. 1 from Hintermüller et al. where the sample fluid flows from the top inlet (yellow) and the reference fluid from the bottom inlet (blue):
(a) Different viscosities but same flow rates
(b) Same viscosities but different flow rates
(c) Repositioning the interface to the centre by varying the ratio of the flow rates with fluids of different viscosities.
Hintermüller et al. investigated the interface detection by capacitive means by measuring different water/glycerol mixtures and comparing them to optical images of the interface position. The flow of both fluids was introduced using Cellix’s ExiGo syringe pumps where the flow rate of the water was increased progressively causing the interface to move from bottom to top. Capacitance was measured using an LCR meter at 10kHz.
Capacitance results for the measurement of 50/50 wt% water/glycerol mixture illustrate the corresponding interface positions obtained by the optical reference measurements, as shown in Fig. 6 from Hintermüller et al.
If you’re considering options for your microfluidic set-up for co-flow or multilaminar flow experiments, Cellix offers both ExiGo syringe pumps and UniGo or 4U pressure pumps. The following table provides a comparison of specifications which should help you choose the right microfluidic pumps for your experimental set-up. The good news is that Cellix’s SmartFlo software can control up to 4 ExiGo syringe pumps and UniGo pressure pumps in one set-up so this gives you greater flexibility – you can have the best of both worlds!
If you have any questions or would like more information, please contact us now, we’d be happy to help.
 Marcus A. Hintermüller et al. A Microfluidic Viscometer With Capacitive Readout Using Screen-Printed Electrodes. IEEE Sensors Journal, Volume 21, Issue 3, 2021, Pages 2565-2572, ISSN 1558-1748. doi.org/10.1109/JSEN.2020.3024837.
 Janet L. Lane et al. Viscosity Measurement:So Easy, Yet So Difficult. Machinery Lubrication Magazine. [internet].
 Siddhartha Gupta et al. Microfluidic viscometers for shear rheology of complex fluids and biofluids. Biomicrofluidics vol. 10, no. 4, Jul. 2016, Art. no. 043402. doi: 10.1063/1.4955123.